Boreholes drilled to gain access to underground hydrocarbon-bearing formations are typically lined over most of their length by steel casing. If tests are to be carried out on a hydrocarbon-bearing formation, or oil or gas is to be extracted from the formation, test or production tubing is lowered into the borehole, and fluid communication with the surface is achieved through the tubing. Conventionally, the tubing is located relative to the casing, and the annulus between the casing and the tubing sealed, using one or more expandable or inflatable packers. Such packers are set when the tubing is in position in the borehole by, for example, inflating the packers with pressurised well fluid. Such setting operations may be time-consuming and often encounter difficulties. Further, the tubing consists of a plurality of threaded sections and the tubing must be tested for pressure integrity as the sections are made up and lowered into the borehole. Such "completion" testing is achieved by pressurising the tubing using well fluid, which may result in inflation and premature setting of the packers.
It is among the objects of embodiments of the present invention to provide an apparatus and a method for sealing and locating tubing in casing which obviate or mitigate the above-mentioned disadvantages.
The section of casing or liner which intersects the hydrocarbon-bearing formation is initially solid, to prevent the production fluid from flowing into the bore until the production tubing is in place and all of the associated apparatus and systems have been prepared. The liner is perforated by explosive charges or guns, typically spaced individual charges which are lowered into the bore and detonated at an appropriate location. The charges may be lowered into the bore on electric wireline, slickline or coiled tubing. However, as the length of the perforating guns which may be used is limited by the depth of the safety valve in the wellbore, and the length of liner to be perforated is generally longer than this depth, a perforating operation will tend to involve a number of runs and thus is relatively time consuming. Further, it is desirable to carry out "underbalanced" perforating, in which the pressure within the wellbore is lower than the formation pressure such that, following perforation, the debris produced by the perforating operation is washed out of the wellbore by the higher pressure formation fluid. In the absence of such a pressure differential the debris may be pushed into the perforations, restricting the flow of production fluid into the wellbore. When carrying out a perforating operation using wireline, slickline or coiled tubing which requires a number of runs only the first perforating operation may be underbalanced.
Guns have been mounted on the lower end of production tubing, thus reducing the need for separate runs and separate perforating operations. However, the remains of the charges and firing arrangements which occupy the wellbore following the perforating operation reduces the internal area of the tubing, thus reducing the production capability of the well. This debris may be milled out, such that it falls to the bottom of the well. However, to accommodate the debris from several thousand meters of perforating guns requires the drilling of a substantial extra section of bore, which may take several weeks' drilling, adding substantially to the drilling cost for the wellbore.
It is among the objectives of embodiments of the present invention to obviate or mitigate these difficulties.